Activation of Toll-like receptors induces dimerization and the recruitment of the death domain (DD)
adaptor protein MyD88 into an oligomeric post receptor complex termed the Myddosome. The
Myddosome is a hub for inflammatory and oncogenic signaling and has a hierarchical arrangement
with 6–8 MyD88 molecules assembling with exactly 4 of IRAK-4 and 4 of IRAK-2. Here we show that
a conserved motif in IRAK-4 (Ser8-X-X-X-Arg12) is autophosphorylated and that the phosphorylated
DD is unable to form Myddosomes. Furthermore a mutant DD with the phospho-mimetic residue
Asp at this position is impaired in both signalling and Myddosome assembly. IRAK-4 Arg12 is also
essential for Myddosome assembly and signalling and we propose that phosphorylated Ser8 induces
the N-terminal loop to fold into an α-helix. This conformer is stabilised by an electrostatic interaction
between phospho-Ser8 and Arg12 and would destabilise a critical interface between IRAK-4 and MyD88.
Interestingly IRAK-2 does not conserve this motif and has an alternative interface in the Myddosome
that requires Arg67, a residue conserved in paralogues, IRAK-1 and 3(M).